The OpenSSL address implementation in Socat 1.7.3.0 and 2.0.0-b8 does not use a prime number for the DH, which makes it easier for remote attackers to obtain the shared secret.
In the OpenSSL address implementation the hard coded 1024 bit DH p parameter was not prime. The effective cryptographic strength of a key exchange using these parameters was weaker than the one one could get by using a prime p. Moreover, since there is no indication of how these parameters were chosen, the existence of a trapdoor that makes possible for an eavesdropper to recover the shared secret from a key exchange that uses them cannot be ruled out.
